docs: empty alias with importas

[ccoVeille]

I discovered the feature of enforcing empty aliases with

• Using `importas` linter to require no alias to be provided for certain packages #5221

A fix was provided

• importas: allow multiple empty aliases #5222

I think it could be interesting to document it.

[ldez]

The reference file should be (when possible) exhaustive about options but not about usage.
This is not exactly a source of examples or "how to".
We should limit them or the linter section will quickly become unreadable (or more unreadable).

So I reduced the example to only one explicit empty alias.

[ccoVeille]

Sure, no problems

[ldez]

LGTM